This research presents piston expander for low-pressure (1-10 bars) power generation to use with organic Rankine cycle and compressed air storage system. The mathematical model is analyzed to show behavior of main parameters such as pressure ratio and charge discharge volumes where these parameters influence to the work and an isentropic efficiency base on dimensionless analysis. The main contributions of this work are method to find the main parameters at the best performance, modification of diesel engine to use as expander and performance evaluation. From the simulation, we found that the high discharge volumes lead to high producing work and isentropic efficiency. The expander was tested to find the power output and isentropic efficiency of the proposed method including torque, speed, temperature and flow rate compared with the simulation results. From the experimental results, the modified expander by the proposed method can be operated by using low pressure working fluid. The engine produces maximum power output are 160 W, 257 W and 431 W at averaged speed of 683 rpm where pressure ratios are 3, 4 and 5. The averaged isentropic efficiency is 57%. (C) 2017 Elsevier Ltd. All rights reserved.